The invention relates to processing of analog signals. In particular, the invention relates to analog-to-digital conversion of signals using timestamps of characteristic signal events.
Analog signals are generated by and/or used in a wide variety of devices and systems. In many of these systems, the analog signals serve as a means of transferring information from one portion of the system to another. Devices that make up systems employing analog signals function to generate, modify, receive and/or detect the analog signals. Examples of systems or devices that make use of analog signals include sensors for monitoring environmental or other system conditions and a wide variety of different communications systems.
In many practical situations encountered in the real world, it is necessary or at least desirable to transform analog signals into a digital representation. This is especially true in cases where digital methodologies are used largely to process and analyze the analog signals. For example, most manufacturers of integrated circuits (ICs) employ some form of automated test equipment (ATE) to test the IC products being manufactured. While ATEs are overwhelmingly implemented based on digital technologies, many of the modem ICs that are being manufactured and tested, produce or use analog output signals. This has become particularly true as modern system-on-a-chip devices are transitioned from the concept to the product phase. The problem for the designers and users of ATEs is how to transform analog signals into a format that can be utilized by the digital ATE. A related problem is the reconstruction of the analog signal from the digital representation.
The conventional approach to converting an analog signal into a digital representation is to use an analog-to-digital converter (ADC). Conventional ADCs sample the amplitude of the analog signal or waveform at successive, regularly spaced, points in time. The sampled amplitude values are converted to a digital format (i.e., digitized) by one of several approaches well known in the art. Once digitized, the analog signal is represented by a sequence of digital values representing the amplitudes sampled by the ADC. Normally, timing of the digital values in the amplitude sequence is known implicitly from the conversion scheme being used. Among the commonly employed ADC approaches known in the art are the over-sampling converters, such as the delta-sigma modulator-based ADCs, the successive approximation ADCs, and the so-called flash ADCs. Each of these technologies ultimately produces a string of digital words, each word representing a sampled amplitude value in digital form, in a time sequence at regularly spaced time intervals.
The analog signal can be reconstructed from the digital words produced by the conventional ADC using a digital to analog converter (DAC). The DAC xe2x80x98readsxe2x80x99 or processes each successive digital word in the time-sequence and produces an analog voltage level at a DAC output port that corresponds to each of the words. By reading the digital words in a manner that is consistent with the order and timing of the original analog-to-digital conversion, the DAC can accurately reconstruct the analog signal.
While conventional amplitude sampled analog-to-digital conversion or amplitude sequence analog-to-digital conversion can provide high fidelity conversion of analog signals to a digital form, the conventional ADCs can be costly to implement in some instances. In particular, many of the conventional ADC technologies are not well suited for simple, accurate on-chip implementations. This is especially true when considering on-chip conversion of analog signals for built-in-self-test (BIST) purposes or in design for test (DFT) instances used in conjunction with an external digital ATE. Similarly, the use of conventional ADC approaches as an interface between an analog device and an ATE can pose many problems, not the least of which is the need for extra dedicated resources in the ATE to accommodate the often high data rate digital signals generated by a conventional time-sampling ADC. Finally, the bandwidth of many conventional ADCs is severely limited by the circuitry necessary to affect the analog-to-digital conversion, especially when many bits of amplitude accuracy are desired.
Accordingly, it would be advantageous to be able to convert an analog signal into a digital representation that preserved selected or specific characteristics of the analog signal and that could optionally provide for accurate signal reconstruction from the digital representation. Such a transformation would solve a long-standing need in the area of analog-to-digital signal conversion, especially as the conversion relates to processing and testing of analog signals by digital systems, such as ATEs and communications systems.
The present invention preconditions an analog signal and converts the analog signal into a digital timestamp representation of the preconditioned signal. The digital representation produced by the present invention effectively records as timestamps the time of occurrence of a plurality of amplitude events within the preconditioned analog signal and by extension, the original analog signal.
In one aspect of the invention, a method of converting an analog signal into a digital representation is provided. The method of converting comprises preconditioning the analog signal to generate a preconditioned signal. The method further comprises producing the digital representation from the preconditioned signal. The digital representation comprises a sequence or set of timestamps. A timestamp is a time record of an amplitude equality event between an amplitude of the preconditioned signal and an amplitude of a reference signal.
In another aspect of the invention, an apparatus for converting an analog signal having an amplitude value to a digital signal is provided. The apparatus comprises a preconditioner that receives the analog signal from an apparatus input and produces a preconditioned signal. The apparatus further comprises a reference signal source that generates separate reference signals of a quantity of N reference signals, where N is an integer equal to or greater than 1. A reference signal may be generated for a finite period of time, depending on the embodiment. The apparatus further comprises a comparator connected to receive the preconditioned signal from the preconditioner. The comparator being further connected to receive the generated reference signals from the reference source. The comparator produces a digital signal at a comparator output. The comparator output is connected to an apparatus output. The produced digital signal comprises a set of timestamps.
In yet another aspect of the invention, a system for converting an analog signal into a digital representation is provided. The system for converting comprises an analog to digital conversion apparatus having an input connected to receive the analog signal, and a quantity N of apparatus outputs, where N is an integer equal to or greater than 1. The analog to digital conversion apparatus preconditions the analog signal and produces a quantity N of separate digital signals at respective apparatus outputs. A digital signal comprises a set of timestamps. The system further comprises a quantity N of transition interval analyzers (TIA). A TIA has an input and an output, wherein a respective apparatus output is connected to the input of a respective TIA. The TIA encodes timing of logic transitions in the digital signal as a sequence of digital words.
Certain embodiments of the present invention have other advantages in addition Is to and in lieu of the advantages described hereinabove. These and other features and advantages of the invention are detailed below with reference to the following drawings.